Considering the current interest in the exploration of the sea, a need has come about for more sophisticated underwater monitoring equipment. Specifically, there has been a long felt requirement for an omnidirectional hydrophone capable of braodband noise measurement while having a self-noise level of at least 10 dB below that of sea state zero.
The heart of the hydrophone is the sensing transducer, and therefore the sensitivity of the hydrophone essentially depends upon the particular construction of the transducer.
Artificial piezoelectric materials such as barium titanates, lead zirconates, etc. provide an excellent sensing material and have greatly advanced the studies in underwater research.
Many techniques of assembling the piezoelectric sensor have resulted in the increased sensitivity however, the present invention further advances the sensitivity by using two crystals, each supporting the other, so that both crystals are exposed to the same acoustic pressure.
The present state of the art of hydrophone low-noise preamplifiers requires that the open-circuit voltage level of a piezoelectric transducer be in the range of -185 to 180 dB re 1 volt per micropascal -85 to -80 dB re 1 volt per microbar) to enable the self-noise level of the total hydrophone to be sufficiently low.